It is known that a vertical magnetic recording system in which a magnetic record medium is magnetized in the direction of its thickness lends itself to high density recording since the smaller the wavelength of magnetization which is magnetically recorded (or the higher the frequency), the smaller in principle is the self-demagnetizing field within the magnetic record medium.
To perform such a vertical magnetic recording, it is necessary to provide a magnetic record medium which exhibits an increased vertical magnetic anisotropy and a magnetic record head which products a sharply defined, strong vertical magnetic field component. The vertical magnetic recording system can be categorized into an auxiliary pole excited type and a main pole excited type.
In a vertical magnetic head system of an auxiliary pole excited type, a main pole 1 (see FIG. 1) comprising a high permeability magnetic thin film as may be formed by Co-Zr-Nb is disposed in opposing relationship with an auxiliary pole 2 comprising a high permeability magnetizable block 2a such as may be formed by ferrite around which a winding 2b is disposed for passing an electric current representing an information signal, with a given air gap 3 interposed therebetween, as shown in FIG. 1. When magnetically recording or reproducing on or from a magnetic record medium 4, the medium 4 is passed through the air gap 3 in a manner such that the magnetizable film of the medium 4 is in sliding contact with the main pole film 1. The medium 4 comprises a flexible base 4a which may be formed by a high polymer film on which a high permeability magnetic layer 4b as may be formed by Fe-Ni, permalloy or the like is formed, which is in turn overlaid with a vertical recording magnetic layer 4c which exhibits a preferred axis of magnetization perpendicular to tho surface of the medium.
Describing the operation of the vertical recording magnetic head of auxiliary pole excited type as mentioned above, an information signal which is delivered from a signal generator, not shown, is applied to the winding 2b, whereupon a magnetic field is produced by the auxiliary pole 2, and this magnetic field acts on the main pole 1 to cause a sharply defined magnetic field to be produced around the tip of the main pole 1. The sharply defined magnetic field is effective to record a magnetic version of the information to be recorded in the medium 4. In the vertical recording magnetic head system of auxiliary pole excited type, the flow of magnetic flux follows an open magnetic path, and thus the sharply defined magnetic field passes through a magnetic path including the main pole 1.fwdarw.magnetic record medium 4.fwdarw.auxiliary pole 2, for example, and the passes through the air space to return to the magnetic pole 1.
It will be understood that the open magnetic path as mentioned above is likely to be influenced by external fields such as that developed by a motor which is used to drive the medium 4 or earth magnetism (which is said to be of the order of 0.3 to 0.4 Oersted in Japan). Thus, assuming that such an external magnetic field is applied to the medium 4 in a direction perpendicular thereto, as indicated by a solid arrow Y shown in FIG. 1, the magnetic field which extends through the main pole 1, the medium 4 and the auxiliary pole 2 will be as indicated by arrows in thin lines. Accordingly, when an external magnetic field of an increased strength as mentioned above is applied when a magnetic recording of a series of information signals has been completed and such signal is then being reproduced, the output level of the reproduced signal will diminish due to the magnitude of the external magnetic field.
An actual measurement of such phenomenon is illustrated in FIG. 2. Specifically, in the vertical recording magnetic head of auxiliary pole excited type as illustrated in FIG. 1, a perpendicular external field of strengths having several different values is applied to the magnetic record medium. An output level which is reproduced in the absence of an external field is plotted as "1" on the ordinate while the external field, represented in Oersteds (Oe) is taken on the abscissa. As shown, when the external field has a magnitude equal to or greater than 1 Oe, the reproduced output level will be substantially reduced to zero.
In addition to causing a reduction in the reproduced output, the influence of an external field also results in a reduction in the recorded level or a diminishing effect upon the remnant magnetization of the medium in the absence of any record/playback operation and when a magnetic head is moved close to or in sliding contact with the medium which has previously been recorded. The occurrence of such influence of the external field is believed to occur for the following reason:
FIG. 3 graphically shows the B-H response of the main pole of a vertical recording magnetic head while FIG. 4 illustrates the B-H response of a record medium which is magnetized using a vertical recording magnetic head which exhibits the response as indicated in FIG. 3. In the absence of an external magnetic field, the main pole will be magnetized to a flux density corresponding to a saturation level .+-.Bs in both positive and negative directions or in opposite directions from an origin 0, varying with an alternating magnetic field produced by a record signal, in a manner as illustrated in FIG. 3. On the other hand, the record medium will be magnetized to a flux density corresponding to a saturation level in both positive and negative directions, by the magnetic field developed by the main pole which is magnetized in the manner mentioned above, as illustrated in FIG. 4, thus retaining a remnant flux .+-.Br. This represents a normal magnetic recording in the record medium. In the presence of an external magnetic field (in the positive direction which may be caused by the earth's magnetism or by a magnet of a d.c. motor), the magnetizing force or magnetic field which acts upon the main pole comprises an additive effect of the magnetic field produced by the recording signal and the external magnetic field. In other words, the magnetizing force will shift in the positive direction on the abscissa, as viewed in FIG. 3. As a consequence, the operating point for the magnetization (flux density) of the main pole will move to a point A shown in FIG. 3, with the consequence that while it may be magnetized to the saturation level Bs in the positive direction, it may not be magnetized to the saturation level -Bs in the negative direction. When the recording medium is magnetized by a magnetic field from a main magnetic pole having a biased magnetization, the remnant flux in the recording medium may retain +Br in the positive direction, but the remnant flux in the negative direction will be reduced to -Bk (see FIG. 4), which is smaller in magnitude than -Br. In other words, the external magnetic field has caused a reduction in the magnetic recording level.
When the vertical recording magnetic head is brought close to or in sliding contact with the recording medium having a biased magnetization as mentioned above, in a manner illustrated in FIG. 1, and when an external magnetic field of a magnitude which shifts the magnetization in the main pole to the point A shown in FIG. 3 is applied, the magnetizing force which results from the external field (principally concentrated on the main pole) causes a change of the magnetization in the recording medium from its initial remnant magnetization -Bk to a value corresponding to a point A' shown in FIG. 4. If the external field is removed subsequent to a change which has occurred in this manner, thus removing the magnetizing force from the main pole, the magnetization in the recording medium cannot be restored to its original remnant magnetization -Bk, but will revert to a value -Bk', which is smaller in magnitude (see FIG. 4). This is because there is an irreversible relationship between a magnetization (flux density) and a change in the magnetic field (magnetizing force) as is well recognized.
Thus, the remnant magnetization or magnetic recording level will diminish in response to the application of an external magnetic field to a recorded medium with or without subsequent removal of the external field. It will be readily appreciated that such tendency will be notable particularly when the main pole of the vertical recording magnetic head is brought close to or into sliding contact with the recording medium.
Assuming that a normal magnetic recording has been made on a medium it is then desired to reproduce recorded information from the medium in the presence of an external field, which is a d.c. field as mentioned previously. In this instance, the external field will be additive to a magnetic field which is produced by the recorded medium, so that the operating point for the main pole of the vertical recording magnetic head will move in the positive direction on the abscissa from the origin O to the point A, for example, or to a point B if the external field has a greater magnitude. It will be apparent from the B-H response curve shown in FIG. 3 that when the operation is centered about the point A or B, the rate of change in the magnetization or the flux density in the main pole which corresponds to a change in a magnetizing force, which now comprises the signal field from the medium added with the external field, will be greatly reduced, and will be substantially equal to zero in the event the operating point is shifted beyond the point B in the positive direction. In this manner, the sensitivity of reproduction will be degraded.
A reduction in the recording/reproducing level under the influence of an external magnetic field, and a reduction in the recorded level of the medium when a magnetic head is brought close to or into sliding contact with a recorded medium occur not only in the vertical recording magnetic head of auxiliary pole excited type, but also in the magnetic head of main pole excited type. Thus, it is recognized that the vertical magnetic recording scheme is susceptible to the influence of at external magnetic field, to a greater degree than that which occurs in a conventional parallel-to-surface recording scheme.
While a shielding technique which prevents an electromagnetic interference (see, for example, M. Mogi "Remedies against Noises and Troubles in Electronic Circuits", published from CQ Shuppan-Sha) or a shielding technique which prevents the occurrence of electrostatic noise (see Laid-Open Patent Application No. 186,112/1984) are well known in the art, there is no established technology which overcomes the described problems inherent in vertical magnetic recording.
Accordingly, it is an object of the invention to provide a vertical recording magnetic head which prevents an influence of an external magnetic field, which would reduce the recording/reproduced output level or the recorded level of the medium when the magnetic head is brought close to or into sliding contact with the recorded medium.